The aim of the present invention is to modify an alternating distributor of the type specified at the outset in such a way that despite a constantly alternately closed product valve the product stream flows completely uniformly through the central distribution chamber and the second open product valve without formation of dead spaces, in particular in the distribution chamber. Furthermore, cases of decomposition or cross-linking of product residues in the valve closed at any time should be prevented as far as possible.
This task is solved according to the invention by an alternating distributor of the category identified at the outset which is characterised in that
the two product valves (3) are in a Y-shaped arrangement relative to product line I (2), characterised in that the centre lines of the two product valves (3) run at an angle of 100 to 140xc2x0 to one another and at an angle of 50 to 70xc2x0 to the centre line of product line I (2),
the valve block (8) possesses a mushroom-shaped displacer headpiece (11) which when the valve is closed is positioned fully in the central distribution chamber (1) with formation of a gap (i) relative to the wall of the distribution chamber allowing product flow and whose shaft (12) when the valve is open is positioned at the level of the branching of product line II (6),
and the central distribution chamber (1) including the displacer headpiece (11) positioned in it possesses only surfaces and edges which run at an obtuse angle to the flow of product with exclusion of surfaces or edges arranged perpendicularly to the product flow.
Preferably in the region of the housing enlargement (5) the product valve housing (4) has a valve seat (13) in which the valve block of a valve piston of an auxiliary valve (10) in the closed position engages, said valve block being movable inside a housing constructed as a guide cylinder, characterised in that the housing of the auxiliary valve (10) is optionally connected to sealable lines (15) for venting and/or product discharge and/or for feeding in auxiliary agents such as air or nitrogen or steam.
The present invention distinguishes itself in that it avoids the disadvantages of normal changeover or alternating valves. The alternating distributor is far superior to the conventional valves. In order to obtain its present outstanding properties the alternating distributor was equipped according to the invention with the following special features.
In contrast with known distributors no surface or edge stands transversely at 90xc2x0 to the direction of flow. In this way no jamming can arise at any surface which could prolong product dwell times. All angular positions of the housing walls along the flow path of the product are fixed in such a way that they always form an obtuse or acute angle and in this way no dead spaces can be produced in the flow path. There is no appreciable dead space due to the selected tolerances. The shaping of the housing and piston allows a dynamic flow which is able to sweep uniformly through the entire housing space. Despite the piston contours of the product valves there is a relatively small overall pressure loss in the distributor due to the enlargement of the valve housing at the necessary points.
In the present alternating distributor the piston and the valve seat are constructed in such a way that smooth, jolt-free opening flow characteristics result. The opening characteristics of the valve make it possible to design the distributors to be switched over fully automatically. Apart from the valve housing, the valve seat is a contributory cause of the smooth opening characteristics. Deliberately elongated opening cones in the housing of the distributor and the tolerances selected allow slow opening of the valve without a sudden pressure increase.
The mushroom-shaped displacer headpiece of the closed valve is set in such a way that it projects over the centre line into the opposite pipe space. In this way the mushroom head offers resistance to the straight, short product pathway and forces the main mass of product to slide along the longer path at the housing wall. The obtuse angles favour this product guidance. In this way a perfect flushing effect is produced. In order to reduce the relatively high pressure loss of the mushroom shaft and at the same time to exert an influence particularly favourable to flushing of the wall the diameter at the base of the shaft was reduced as far as possible. In this way the relatively large mushroom head shifts the flow further towards the base of the mushroom. This design feature also serves to improve the guidance of the product flow along the longer outside wall of the housing.
The housing was designed in such a way that the flow of polymer is guided. The pressure differences with the installed valve piston are devised in such a way that flushing of all corners and edges is ensured. The positioning of the product valves relative to one another is optimised in such a way that optimum flow without flow shadows is ensured for the flow of polymer through the distributor space. The angular positioning of preferably 120xc2x0 relative to one another is an optimum with reference to the overall production of the distributor design. The range possible for the angular positioning is 100 to 140xc2x0.
The product valves can be equipped in inherently known manner with auxiliary valves for venting and/or discharging and/or feeding in auxiliary agents. The auxiliary valves are preferably arranged in such a way that after operation they can drain out. This is particularly advantageous when the high-viscosity liquid is a polymer melt which tends to carbonise. Tangential fitting of the auxiliary valve on the product valve is suitable in many cases.
The alternating distributor according to the invention can be used for any viscous liquid which is to be distributed from a single supply line alternately over two outlet lines of vice versa. A preferred application is the distribution of a polymer melt. Examples of such polymers are polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polyamide 6, polyamide 6.6, polycarbonates, polyolefins and copolymers thereof. In this case the distributor and the polymer lines must be enclosed by one or more heating jackets in which a suitable heat transfer medium is circulating. Other types of heating, band heaters for example, may likewise be used.
Polymers exhibit the unpleasant property of decomposing in a hot pressureless space into low-viscosity and high-viscosity substances. While the low-viscosity products become gaseous over time and disappear into the atmosphere, the high-viscosity products carbonise. This formation of carbon results in the end in total blockage of the lines and valves. If a valve in the alternating distributor is shut off and not used for a relatively long period it is advisable to carry out a hydrolysis operation to clean the valve. The hydrolysis prevents carbonisation of the product in the valve. Before hydrolysis can be started the polymer has to be completely drained of polymer. The steam is not able to push the polymer out since it is preferably at a pressure of 1 to 2 bar absolute, and by particular preference 1.0 to 1.3 bar. The steam for hydrolysis is advantageously fed in via the auxiliary valves fitted in the base structure of the product valves. By this means hydrolysis can begin directly at the seat of the polymer valve. Care must be taken for the safe removal of the steam which may be laden with oligomeric hydrolysis products. Depending on whether the auxiliary valve or the product valve is subjected to hydrolytic cleaning the removal of the steam ensues via a venting nozzle on the auxiliary valve or via a corresponding emptying aperture in product line II.
The invention is discussed below in more detail with reference to drawings wherein: